Preparation of sols



Patented Dec. 18,, 1945 PREPARATION OF SOLS Morris D. Marshall, Arlington, Masa, assignor to Monsanto Chemical Company, St. Louis, Mo., a corporation of Delaware No Drawing. Original application April 30, 1942, Serial No. 441,212, now Patent No. 2,356,773, dated August 29. 1944. Divided and this application June 28-, 1944, Serial No. 542,629

3 Claims. (01. 252-309) This invention relates to the preparation of sols, and particularly to an improved method of Q preparing relatively stable and substantially salt free aqua-organosols of unusually high colloidal oxide content.

According to prior methods it has been possible to form sols having a fairly high colloidal oxide content, but these methods have not been very satisfactory, particularly for commercial operations. Silica aquasol, for example, has been prepared by reacting an alkali silicate with a mineral acid, removing the inorganic salt from the resulting solution by dialysis, and then con-' centrating the remaining aquasol by evaporation. This, however, is a long and drawn out process, primarily because salt removal by di-.

alysis is slow. Moreover, dialysis requires delicate handling and skilled workmanship, which interferes to a large extent with successful manufacture on a commercial scale. It is also not possible by this method to obtain sols which are substantially free of salt and which at the same time have a silica content higher than about 2Q% by weight.

It is accordingly 8. primary object of this invention to provide methods of making substantially salt free sols which contain an exceptionally high concentration of a colloidal inorganic oxide (as high as 40% by weight) and are stable over relatively long periods of time.

A further object of the invention is to provide methods of making sols of the type described in which th volatile component is predomi-. nantly water, and an upper layer in which the volatile component is predominantly organic. The lower layer contains practically all of the colloidal oxide, and is in the form of an aquaorganosol, which expression will hereinafter be used to distinguish the sols prepared as described herein from organo-aquasols, such as the intermediate sol referred to above, which contains a preponderant amount of organic solvent. The lower layer may be separated from th upper layer by drawing off, or by decantation of the upper layer, or by any similar operation.

In the preparation of the initial aquasol the methods for preparingaquagelsor hydrogels described in the Kistler Patent No. 2,093,454 may be followed, but gelation is prevented by proper control of pH. Thus, when preparing silica aquasol, for example, by admixing amineral acid and an alkali silicate solution, it is essential to operate on the acid side, for most purposes at a pH between 1.5 and 4, but preferably between 1.5 and 3. This makes possible a long period bethe gel,

tween the sol formation and the precipitation of for example, as long as or 40 days. Where it is desirable, however, the preparation of the aquasols may be carried out at a pH as which methods are thoroughly adapted for large scale operations.

Still further objects and advantages of the invention will appear from the following description and appended claims. Before explaining in detail the present invention, however, it is to be I understood that the invention is not limited in its application to the details described herein, since the invention is capable of other embodiments and of being practiced-or carried out in various ways.

The invention is carried out in general by first preparing an aquasol containing a colloidal inorganic oxide and a dissolved inorganic salt, then adding to the aquasol a water-miscible organic solvent in an amount sufficient to precipitate the inorganic salt, removing the precipitated salt from the resulting organo-aquasol, as for example by filtering, centrifuging, decantation, or the like, and finally adding'to the organo-aquasol a suflicient quantity of a water-immiscible organic solvent, usually about an equal volume, to cause the formation of two layers, i. e., a lower layer high as 6, since then also there is an appreciable period between sol formation and gelation, thus.

permitting removal of the inorganic salt formed.

A wide variety of both water-miscible and water-immiscible organic solvents may be used according to the invention. Suitable water-miscible organic solvents,'for example, include ace tone, methyl alcohol, ethyl alcohol, isopropyl al cohol and normal propyl alcohol. Suitable water-immiscible organic solvents include higher alcohols, such as the butyl or amyl alcohols; alkyl acetates, such as ethyl, butyl or amyl acetate; ethers, such as methyl or ethyl ether; and hydrocarbons. such as lactol spirits and toluene.

A further understanding of the invention will be obtained from the following examples:

Example I silica and dissolved sodium sulfate. Sixteen hundred (1600) pounds of acetone are then added.

60% of the water originally present, to separate as a lower layer. This is either drawn ofl at the bottom, or the upper layer is removed by decantation. The resulting aqua-organosol is substantially pure, i. e., free of salt, and has a concentration of silica of about 28% by weight.

As a variation of th above, more water may be added to the acetone-aquasol before adding the acetate, in which case a more dilute sol is obtained in the lower layer, since approximately the same amount of water is retained by the upper or acetone-acetate layer even though relatively large variations are made in the total water originally present.

Example 11 Eight hundred (800) pounds of sodium silicate containing approximately 9% NazO and 29% SiOz are mixed at room temperature with 350 pounds of water. This mixture is slowly injected at the point of maximum agitation into 335 pounds of 31% H2804 which has been cooled to C. and which is being stirred by a high speed impeller. The temperature during the mixing is kept below 10 C., and the silicate is added until the pH is about 1.8, or until about 90% of the silicate has been added. Three thousand two hundred (3200) pounds of acetone which has been cooled to 0 C. are next added, slowly at first, then more rapidly.

The resulting mixture, which is in the form of an organo-aquasol and contains precipitated sodium sulfate crystals (Na2SO4-10H2O), is then cooled to 0 C. and settled at a temperature between 0 and C. Approximately 85 to 90% of the slightly opalescent sol is then decanted from the sodium sulfate crystals with theresult that about 99% of the total sodium sulfate formed is removed.

Three thousand five hundred (3500) pounds of butyl acetate are added to 3600 pounds (about an equal amount by volume) of the above sol, after bringing it to room temperature, and the mixture is stirred rapidly for about 2 minutes. A heavy predominantly aqueous layer containing about 98% of the total silica and 60% of the total water (about 28% SiOz by weight) separates immediately. Upon removing this aqueous layer by drawing off and further extracting it with 2700 pounds of butyl acetate an aqua-organosol of about 40% SiOi content is formed.

Example III A silica aquasol containing dissolved sodium sulfate is prepared as described in Example II. Three thousand two hundred (3200) pounds of ethyl alcohol which hasbeen cooled to 0 C. are then added, and the resulting mixture, which is in the form of an organo-aquasol and contains precipitated sodium sulfate crystals (Naz S04 H2O) heavy predominantly aqueous layer separates .immediately, 'and can be, removed by drawing off. An aqua-organosol of exceptionally high silica content may be obtained by further extracting the aqueous layer with an additional quantity of toluene.

While the examples have been limited, in the interests of brevity, to methods of preparing sols of silica,- it will beapparent that salt free aquaorganosols of other oxides may be prepared in a similar manner, that is, by forming the proper salt-containing aquaso'l, precipitating out the inorganic salt formed by adding a quantity of a water-miscible organic solvent, removing theprecipitated salt, and separating the aqua-oreanosol from the greater portion of the water-miscible organic solvent by adding a water-immiscible organic solvent. Moreover, other water-miscible Q ganic solvents may be substituted for the acetone and ethyl alcohol, and other water-immiscible solvents may be substituted for the butyl acetate and toluene described in the examples.

It is also possible by selecting the solvents used in any step, and/or by varying the proportions of the water-miscible organic solvent and the aquasol used; to vary the per cent by weight of silica or other oxide present in the final purified aquaorganosol. In general, it is desirable to use the water-immiscible solvent in an amount by volume substantially equal to the volume of the organoaquasol being treated, as smaller'amounts of the water-immiscible organic solvent, particularly when the ratio of the water-immiscible solvent to the organo-aquasol is decreased by more than about 20%, tend to decrease its. efliciency, while larger amounts of the water-immiscible organic solvent do not appreciably increase its efiiciency or action. However, subsequent additions of the water-immiscible organic solvent to the separated aqueous layer, that is, subsequent extractions of the predominantly aqueous layer, may be made to advantage, using a much smaller quantity of the of the organo-aquasol because of the lower efli- V ciency of the salt precipitation and the danger of gelation. Moreover, temperatures below 0 C. can be used after the addition of the water-miscible organic solvent, if desired, provided the temperature is kept above the freezing range of the mixture.

The relatively stable and substantially salt free aqua-organosols of high oxide content prepared according to this invention may be used for various purposes, but they are, as far as now known, best suited for making relativel dense aerogelu; of the type described in the Kistler Patent No. 2,093,454, as well as relatively dense liquegels or the like. Dried inorganic xerogels, such as driez! silica gel, may be made, for example, by dehydrating 'or drying the sols under atmospheric pressure inthe usual manner. Aerogels are made by autoclaving the sols directly, that is, without first forming a gel. The sols of the present invention may also be used to advantage, however, as binders for sand molds, and are valuable film formers, either alone or in combination with other coatingmaterials. V

. Where reference is made herein to water-miscible and water-immiscible organic solvents, it is to be understood that reference is made to organic solvents of the type commonly employed in varnishes, lacquers and like coating compositions, which solvents, in the case of the watermiscibie ones, act as precipitates for inorganic mixture of hydrocarbons having an evaporation rate similar to or close to that of toluene. The product has a specific gravity of .7313 at 60 F. and the following A. S. T. M. distillation range:

' Temper- Per cent distilled 8mm F. Initial boiling p in 200 507 distm 220 90 distill 240 June 9, 1942, which contain a predominating amount of organic solvent or solvents in the volatile component.

This application is a division of my copending application, Serial No. 441,212, filed April 30, 1942 issued as Patent number 2,356,773 on August 29, 1944, which is a continuation-in-part of my application Serial Number 272,404, filed May 8, 1939, now issued as Patent No. 2,285,449.

What I claim'is: I

l. The method of forming an aqua-organosol composed of colloidal silica, water and a relatively minor amount of organic solvents which comprises forming an aquasol containing colloidal silica and a dissolved inorganic salt, adding a sufficient quantity of ethyl alcohol to precipitate the inorganic salt, removing the inorganic salt precipitated thereby, adding a suflicient quantity of a lower alkyl acetate to cause the separation of an aqua-organosol layer, and

- separating the aqua-organosol layer which forms.

2. The method of forming an aqua-organosol composed of colloidal silica, water and a relatively minor amount of organic solvents which comprises forming an aquasol containing colloidal silica and a dissolved inorganic salt, adding a suflicient quantity of ethyl alcohol to precipitate the inorganic salt, removing the inorganic salt precipitated thereby, adding a suflicient quantity of butyl acetate to cause the separation of an aqua-organosol layer, and separating the aquaorganosol layer which forms.

3. The method of forming an aqua-organosol composed of colloidal silica, water and a relatively minor amount of organic solvents which comprises forming an aquasol containing colloidal silica and a dissolved inorganic salt, adding a sumcient quantity of ethyl alcohol to precipitate the inorganic salt, removing the inorganic salt precipitated thereby, adding a sumcieht quantity of amyl acetate to cause the separation of an aqua-organosol layer, and separating the aqua-organosol layer which forms.

MORRIS n. MARSHAIL. 

